1. Field of the Invention
The present invention relates to radar and more particularly to interferometric and holographic radar. The invention can be used to form holograms, to form stacked holograms, to detect motion and vibration within a gated region, and to find range.
2. Description of Related Art
Pulse echo and FMCW high resolution radars typically have emissions that are wideband to ultra-wideband (UWB). UWB impulse radars emit short pulses of ½ to one RF cycle in duration, with corresponding bandwidths extending from 500 MHz to 10 GHz or more. Wideband pulse-echo radars emit bursts of RF sinusoids; tank level sensing radars typically emit 10 to 20 RF sinusoids in a burst with a corresponding bandwidth of greater than several hundred megahertz. Similar bandwidths pertain to high resolution FMCW radars. Operation of these high bandwidth radars is severely restricted by regulatory agencies such as the FCC. Examples of these restrictions include: (1) UWB radars in the U.S. can only be operated outdoors with extremely limited radiated power levels and only when handheld; (2) wideband tank level sensing radars can only be operated inside tanks, and cannot be used to sense river levels, for example; and (3) ISM bandwidth is very limited, such as the 50 MHz wide 10.5 GHz band.
While high bandwidth radar is subject to severe regulatory limitations, operation without a range-gate introduces other severe limitations. It should be noted that the use of a range gate generally infers high bandwidth. Range gating usually requires high spatial resolution, which implies a narrow sampling aperture and matching narrow, high bandwidth emission pulse. Short range radars, and generally, radar sensors, frequently require very high resolution gating.
CW Doppler radars are commonly used to sense motion. However, these radars have no maximum range limit. Undesired moving objects, i.e., clutter at any range can produce a response. The lack of a range gate may be ideal for police speed-sensing radar, but it is completely undesirable for security alarms; a person moving outside a protected zone could false-rigger an ungated radar. A range gate is clearly needed in many applications. Numerous range-gated motion sensing radars exist in the prior art; yet they often require high bandwidths and are thus subject to tight regulatory restriction. A low bandwidth range-gated radar is needed.
A holographic radar is disclosed in U.S. Pat. No. 5,455,590, “Real-Time Holographic Surveillance System,” by Collins et al. An apparatus is disclosed that forms a holographic radar image by scanning an antenna along X and Y axes. However, the radar is a CW radar and has no range gating. Thus, the holographic image can be contaminated by echoes from outside the image plane. It is only practical in situations where the clutter scene can be tightly controlled. Further, it can only work with objects that are not semi-transparent to radar; e.g., forming a holographic image of a wood or plastic surface could be difficult if not impossible. Time gating, i.e., range gating, of the radar echoes is needed. The '590 cannot form stacked holograms, i.e., holograms representing multiple surfaces or slices inside a solid, since there is no time-gating to resolve echoes in the downrange direction.
The prior art lacks: (1) a low bandwidth radar with high spatial resolution range gating; (2) a high resolution radar that can operate in narrow ISM bands; (3) a high resolution radar that is high immune to interference so it can operate in crowded ISM bands; (4) a holographic radar than rejects clutter; (5) a holographic radar that can image semitransparent objects; (6) a holographic radar that can form stacked holographic images; and (7) a narrowband motion sensing radar with close-in range gating.